String And Array

* Examples
- To get the first letter which is not repeated in the string
Build the key index between character and its occurrence with array or hashtable.
- Remove the specified characters in the string
Build the key index between character and its status (stay or removed). In place deleting operation with two pointers.
- Reverse the order of a sequence of words separated with space OR circularly shift the string
In place reverse with single word reverse after getting the start and end of words.
- Conversion between signed integer and string
1) To convert between char 0~9 and its integer with the aid of '0'
2) Accumulate in the conversion of atoi from left to right
3) Separate digits in the conversion of itoa from right to left and so reverse order output
- Merge two sorted arrays with the length of M and N (M>>N)
Binary search in M